Method and apparatus for facilitating signal discrimination in a wireless network by applying known frequency offsets

ABSTRACT

Method and apparatus for demodulation in a system having known frequency offsets. First and second signals from two users occupy substantially the same bandwidth and use substantially the same carrier frequency, while the relative frequencies of the signals on different transmit antennas associated with a base station are adjusted to have known, specific frequency offsets. These known frequency offsets are used at the receiver to aid in estimating any known frequency errors and to demodulate the multiple transmitted signals. In addition to being useful for demodulation, known frequency offsets can be assigned to each of a plurality of base stations to facilitate identification of a particular base station. The approach can be used for any type of time domain multiple access (TDMA) system, including Global System for Mobile (GSM), or for code division multiple access (CDMA) systems.

BACKGROUND OF INVENTION

[0001] The use of multiple transmit antennas in digital, wirelesscommunication systems has been shown to be useful for increasing datarate and for improving performance for receivers with either single ormultiple receive antennas. In these systems, the transmitted signals aretransmitted with substantially the same carrier frequency and bandwidthso that their channel responses overlap in the frequency domain. Manyknown approaches are open-loop systems, i.e. there is no feedbackbetween the receiver to the transmitter regarding knowledge about thechannel response. When this channel knowledge is present, then multipletransmit antennas can be used to improve data rates using closed-looptechniques.

[0002] The advantage in the above techniques is obtained-since thechannels are not the same from different transmit (Tx) antennas to thereceive (Rx) antenna(s). The various approaches rely upon being ablediscriminate the different transmitted signals from one another. Whencoherent reception of the transmitted signals is used, the channelresponse for each transmitted signal is obtained at the receiver via anestimation process. This estimation process is complex, processorintense and less accurate when channels from different transmittedsignals interfere with one another. Additionally, multipath makes theestimation process more difficult since the signals overlap in time(delay) with one another.

[0003] Semi-blind techniques have been used to estimate the channelresponse in order to perform joint demodulation for one desired signaland one interferer. However, in those cases that channel responses forthe two users are similar, the performance of the joint demodulationreceiver degrades compared to the case when channel estimates can beeasily distinguished. One technique for improving performance in fadingchannels is to adaptively update (track) the channel estimates. However,this leads to the problem of channel switching, where estimated channelsare incorrectly assigned to user signals. Even after the fadingwaveforms be distinguishable, channel estimates may be incorrectlyassigned to users. This channel switching results in detection errorsfor long bursts, until the channel estimates are reassigned to theircorrect users, for example, by another channel switch.

SUMMARY OF INVENTION

[0004] The present invention involves the use of known frequency offsetsto facilitate signal discrimination by helping to distinguish signalstransmitted from different antennas of a base station. While the signalsoccupy substantially the same bandwidth and use substantially the samecarrier frequency, the relative frequencies of the signals on differenttransmit antennas are adjusted at baseband or an intermediate frequencyso that there is a small, specific frequency separation between signalstransmitted from the different antennas. This results in each signalhaving a different offset from the carrier, resulting in a set offrequency offsets for the signals relative to the carrier. This set offrequency offsets results in a higher degree of separability inreceivers that perform channel estimation or interference cancellation.Additionally, the frequency offsets can help distinguish signals fromdifferent base stations since different, known sets of frequency offsetscan be assigned to each of a plurality of base stations. The approachcan be used for any type of time domain multiple access (TDMA) system,including Global System for Mobile (GSM) or for code division multipleaccess (CDMA) systems.

[0005] In some embodiments, a base station having at least two antennascommunicates or transmits first and second signals to a mobile terminalby first establishing known frequency offsets to be applied to the firstand second signals. The frequency offsets are known or made known at themobile terminal. The frequency offsets may be agreed as part of thesystem design, or it may be looked up from a data structure or table inthe base station's memory. The mobile terminals may have the set offrequency offsets stored, or the base station may transmit the set offrequency offsets to the mobile terminal over a control channel prior tobeginning normal operation. The base station applies the known frequencyoffsets to the first and second signals, and upconverts the signals fortransmission at substantially the same carrier frequency and bandwidth.In addition to the applied known frequency offsets, there may be unknownfrequency errors introduced by the transmitter. This places requirementson the design of the known frequency offsets, although theserequirements may be relaxed by locking the carrier frequencies of thedifferent transmit chains. The transmitted signals can then bedemodulated at a mobile terminal based at least in part on the knownfrequency offsets.

[0006] A mobile terminal in the system according to some embodimentsreceives the first and second signals from first and second associatedtransmit antennas, and downconverts the signals. The mobile terminaldetects symbols in the first and second signals using channel estimatesand frequency estimates for the first and second symbols based at leaston part on the set of known frequency offsets. At the mobile receiver,frequency estimation is required to correct the unknown frequency errorspresent in the receive signals. One approach is to use automaticfrequency control (AFC) techniques, although, other frequencycompensation methods could be used instead.

[0007] The system according to some embodiments of the invention can bedesigned so that a plurality of neighboring base stations use aplurality of corresponding sets of frequency offsets. Information on thefrequency offsets can be known and programmed into mobile terminals, orcan be transmitted initially to a mobile terminal over a control channeland stored in memory. The base stations each have a memory associatedwith the antennas and transceiver apparatus where frequency offsetinformation is stored. A mobile terminal, which includes a radio block,one or more antennas, processing and control logic, and baseband logic,can use the frequency offset to identify a base station, facilitatingranging and other functions. The various hardware and software ormicrocode in the base stations and mobile terminals form the means tocarry out the various aspects of the invention.

BRIEF DESCRIPTION OF DRAWINGS

[0008]FIG. 1 illustrates a functional block diagram and method ofoperation of a transmitter and receiver operating according toembodiments of the present invention.

[0009]FIG. 2 is a functional block diagram that illustrates furtherdetail of an interference cancellation receiver apparatus and thereceive method using automatic frequency control according to anembodiment of the present invention.

[0010]FIG. 3 is another functional block diagram that illustratesfurther detail of a joint demodulation receiver apparatus and thereceive method using automatic frequency control according to anotherembodiment of the present invention.

[0011]FIG. 4 is a phaser representation of the first and second signalswithout use of frequency offsets according to the invention.

[0012]FIG. 5 is a phaser representation of the first and second signalsaccording to embodiments of the invention to enable separation of thetwo signals.

[0013]FIG. 6 is a functional block diagram of a mobile terminalincorporating a receiver according to embodiments of the invention.

[0014]FIG. 7 is a network diagram of a system in which differentfrequency offsets are assigned to a plurality of neighboring basestations according to some embodiments of the invention.

DETAILED DESCRIPTION

[0015] This invention is described in terms of example embodiments,which take the approach of using different frequency offsets for signalstransmitted from different antennas, but using the same carrierfrequency on each antenna. These embodiments are given by way ofexample, and those of ordinary skill in the art will recognize thatthere are other embodiments, which may be implemented without departingfrom scope of the appended claims.

[0016] The embodiments disclosed involve both transmission and receptionof signals in accordance with the invention. In the example embodiments,a mobile terminal receives, and a base station transmits. The inventionwill work equally well in reverse, with equipment of the appropriatedesign as will be understood by persons of skill in the art.Additionally, the methods of the invention can be used between twomobile transceivers or two fixed transceivers.

[0017] With respect to some of the mathematical formula and notations,the reader should be aware that subscripts and superscripts sometimescouldn't be used in drawings because they would result in text that maybe too small to read clearly. In some cases, characters which are moreproperly written as subscripts or superscripts are shown in regulartext, for example s₁ (k) might be shown as s1(k). In cases herein wherea drawing is being discussed directly, the notation in the drawing isused. Otherwise, proper mathematical notation may be used. The equationsherein are still understandable to those of skill in the art.

[0018] In multiple-input multiple-output (MIMO) systems, data rate isincreased without substantially increasing the bandwidth of thecomposite transmitted signal. The drawback is that the transmittedsignals interfere with each other requiring a more complicated receiverto separate the signals. To facilitate this separation, with simplerreceivers, different frequency offsets are used together with afrequency estimation technique to help distinguish the signals at thereceiver. Estimation techniques include joint frequency estimation ormultiple-user phase-locked loop approaches as are known in the art.

[0019] Furthermore, mobile terminals often receive transmissions frommultiple base stations. The frequency offsets themselves can be used tohelp distinguish which signals are transmitted from which base stations,by assigning a unique set of known offsets to each of a plurality ofneighboring base stations, each having at least two antennas. A mobileterminal can then use this information in detecting the desiredtransmitted signal or rejecting the signals from interfering basestations. Additionally, these unique frequency offsets may be used toestimate other important characteristics of the transmitted signals. Forexample, the received signal strength may be used to assist inperforming soft-handovers, and more accurate signal strength values maybe obtained when the signals can be assigned to known base station.Also, the identification of known base stations can be used for mobileranging and positioning.

[0020] One embodiment of the invention is shown by the block diagram inFIG. 1 of system 100, which shows the desired signal transmitted withtwo transmit (Tx) antennas, 102, and 104 and received by two receive(Rx) antennas, 106 and 108. original signal is first coded andinterleaved and sent to a space-time coding and modulation unit, 110.The purpose of this block is to map the modulated signals onto thedifferent Tx antennas. For example, in the so-called “V-BLAST” scheme,different symbols are modulated and mapped onto different transmitantennas directly (i.e. without additional space-time coding). TheV-BLAST scheme is described in Foschini, G. et al., “SimplifiedProcessing for High Spectral Efficiency Wireless Communication EmployingMulti-element Arrays,” Journal on Selected Areas in Communications, 17(11):1841-1852, November 1999, which is incorporated herein byreference.

[0021] Transmitter RF units 111 and 112 upconvert and then transmit thesignals. After the signals are transmitted over the channel and receivedat the Rx antennas, the signals are downconverted at receiver RF units113 and 114 and sent to a demodulation unit. The demodulation unitperforms channel and frequency estimation in order to coherently detectthe transmitted signals. After demodulation, the resulting detectedsignals are passed to a decoder unit, which detects the original inputbits.

[0022] In FIG. 1, signals s1(k) and s2(k) have frequency offsets f₁ andf₂ applied at 115 and 116, respectively. Random frequency errors arealso present, and are shown modeled in the same way at 117 and 118,respectively, as f_(T1) and f_(T2). These random frequency errors occur,for example, due to the tolerances of manufactured clock circuitry,environmental conditions, and similar factors. In the case of tworeceive antennas as shown in FIG. 1, received signals r1(k) and r2(k)each contain a contribution from the first and second transmittedsignals, s1(k) and s2(k) as indicated by the arrows between the transmitand receive antennas. In the case of a receiver with one antenna, asingle received signal r(k) contains a component from both s1(k) ands2(k).

[0023] It should be noted for the above approach that detectionperformance in the steady-state period of the AFC may be better whenthere is a significant frequency difference between the two signals.This occurs, in effect, since the ability to separate the two signalsrelies upon differences in the channel responses between users. Thus, ifthe difference between the additional offsets caused by random systemerrors, /f-f_(T1)/ is large enough, this adds an extra dimension whichto make the channels different. Systems have been proposed for some timethat rely upon differences in the underlying channel responses in orderto obtain their improved performance. Adding the known frequency offsetsto each transmit chain improves the system as described above. Forreceivers that use multiuser AFC loops, faster convergence during thetransient period is obtained by separating the frequencies of thedifferent transmitted signals so little or no ambiguity exists betweenthem. Second, better detection performance is obtained during thesteady-state period by reducing the probability of having similarchannel responses over a data-burst.

[0024] Returning to the 2×2 MIMO system of FIG. 1, frequencies f₁ and f₂are applied to the first and second transmit signals, respectively, toachieve the known offsets. This approach can easily be extended to morethan two transmit antennas. In FIG. 1, the baseband representation isshown. However, the approach can be used for systems at some carrier orintermediate frequency. The two frequencies f₁ and f₂ should have aseparation larger than the maximum of 2 f_(max1) and 2 f_(max2), wheref_(max1)=/f_(T1)/ and f_(max2)=/f_(T2)/ represent the largest expectedfrequency errors on each transmit antenna. In other words, If /f₁-f₂/should be greater than 2max(f_(max1),f_(max2)). If there are M>2transmit antennas, then the above item holds for frequencies f_(m) andf_(n), for any two antennas m and n where f_(max1)=^(|)|f_(T1)| andf_(max2)=|f_(T2)| represent the largest expected frequency errors oneach transmit antenna. In other words, |₁-f₂| should be greater than2max(f_(max1),f_(max2)). If there are M>2 transmit antennas, then theabove item holds for frequencies f_(m) and f_(n), for any two antennas mand n where m≠n. Additionally, random frequency errors are introduced atthe receiver, and are modeled by applying the frequency errors f_(R1)and f_(R2) at 120 and 122, respectively. The demodulation scheme nowdetects symbols in the signals with the known frequency offsets by downconverting and applying frequency and channel estimation, smoothing,etc., in block 119. The values of f₁ and f₂ are known at the receiverand typically stored so that they can be accessed and used to initializethe frequency estimation approach. As previously mentioned, automaticfrequency control (AFC) techniques, described in further detail below,can be used for frequency estimation in block 119. However, othertechniques known in the art can be applied. For example, amaximum-likelihood estimation technique that uses known trainingsequences or pilot channels can be used, or a phase-locked loop (PLL)can be applied in place of or in addition to other traditional AFCapproaches.

[0025] One common approach for detecting signals transmitted by a MIMOsystem is to separately detect a signal transmitted from one antennawhile canceling signals transmitted from the other antennas. This typeof receiver is denoted as an interference cancellation receiver. In thiscase, AFC can be applied to compensate for the frequency error on thedetected signal only. Such is the case with AFC as applied to the systemof FIG. 1, which is shown in FIG. 2 for the first signal. The process issimilar for the second signal. If two or more receive antennas are used,it is desired to frequency lock the receive chains so that,substantially, f_(R)=f_(R1)=f_(R2). According to this embodiment of theinvention, this approach is used for performing AFC for each detectedsignal. The first signal, r1(k) is denoted s1 when being processedwithin the receiver and the second signal r2(k) is denoted s2. In FIG.2, receiver components 200 include an interference canceling, channelestimation, and frequency estimation block 202, and smoothing block 204.f_(err1) is the initial frequency estimate for s1. f_(e1) is an estimateof the frequency offset of s1. For convenience, the notations f_(off1)and f_(off2) are used, where f_(off1)=f₁+f_(T1)+f_(R) andf_(off2)=f₂+f_(T2)+f_(R). Further details on an AFC approach like thatshown in FIG. 2 can be found in U.S. Pat. No. 5,818,093, which isincorporated herein by reference.

[0026] The AFC approach just described is appropriate when the twouser's signals are demodulated separately. The use of joint AFC,however, is critical to coherently detecting both signals jointly. Theuse of joint AFC is illustrated in FIG. 3, as applied in a systemaccording to the invention. FIG. 3 shows receiver components 300, whichincludes a block, 302, for joint detection, frequency estimation andchannel estimation, and two smoothing blocks, 304 and 306. Again,signals are denoted s1 and s2. The joint detection block 302 estimatesfrequency errors and outputs these estimates as f_(err1) and f_(err2).These frequency error estimates are fed into smoothing blocks 304 and306 to estimate total frequency offsets, f_(e1) and f_(e2). With thepresent invention, the frequency offsets f₁ and f₂ are known. Thefrequency estimate f_(e1) is applied to compensate the received signal,reducing signal s1's apparent frequency error to zero, while changingthe apparent frequency error of the other signal tof_(e2)-f_(e1)=f_(off2)-f_(off1). Initial estimates of f_(e1) and f_(e2)can be set to the known frequency offset values f₁ and f₂, respectively.The joint detection block 302 requires the estimation of the apparentfrequencies to be input, as shown in FIG. 3. Further details on a jointAFC method similar to that shown in FIG. 3 can be found in U.S. patentapplication Ser. No. 09/699,920, filed Oct. 30, 2000 by the inventorhereof, which is incorporated herein by reference.

[0027] Finally, it should be noted that the carrier frequencies of thetwo transmitted signals could be locked in frequency. This can beaccomplished by generating the carriers from the same source at the basestation employing the two antennas. If the carriers are locked infrequency, then this has a couple of implications on the design of thetransmitter and receiver. First, the value of /f₁-f₂/ can be madesmaller, since both transmitters will be shifted by f_(T1) (=f_(T2)),thus preserving the frequency difference of the known frequency offsets.Second, if the receiver chains are also locked in frequency, or if thereis only one receive antenna, then f_(e2)-f_(e1)=f₂-f₁, which is known.In this case, while the AFC must still be applied to compensate for theunknown frequency errors, the difference in the apparent frequencycomponents is known and need not be input to the joint detection block302. To illustrate the efficacy of the approach, consider the receivedsignal y₁(k) which is comprised of the two transmitted signals s₁(k) ands₂(k), and corrupted by the channels c₁₁(k) and c₂₁(k), respectively.The received signal in this case is written as

y ₁(k)=e ^(j2π(f) ^(_(T1)) ^(+f) ^(_(R1)) ^() k) c ₁₁(k)s ₁(k)+e^(j2π(f) ^(_(T2)) ^(+f) ^(_(R1)) ^()k) c ₂₁(k)s ₂(k)+n(k).

[0028] The goal is to have f_(T1) and f_(T2) small, or to lock these twofrequencies so that they affect the two transmitted signals in the samemanner. However, the consequence of this goal is when c₁₁(k)=c₂₁(k),then the situation in FIG. 4 occurs at a specific time instant. Namely,since the two channels are the same and the frequency errors aresimilar, then there is ambiguity in the detection procedure. FIG. 4shows this as the phaser representing the channels for the two signalsbeing coincident at point 400. When the channel coefficients c₁₁(k) andc₂₁(k) do not change or vary slowly over time, then the period where theambiguity exists can last for long bursts of data. After addingfrequency offsets f₁ and f₂ to the transmitted signals, the receivedsignal becomes:

y ₁(k)=e ^(j2π(f) ^(₁) ^(+f) ^(_(T1)) ^(+f) ^(_(R1)) ^()k) c ₁₁(k)s₁(k)+e ^(j2π(f) ^(₂) ^(+f) ^(_(T2)) ^(+f) ^(_(R1)) ^()k) c ₂₁(k)s₂(k)+n(k).

[0029] Now, f_(T1) and f_(T2) can be made small or locked together sincef₁ and f₂ have been added to distinguish the two transmitted signals.This gives the situation shown in FIG. 5 for one time instant. Here, thephaser shows that the channel responses are now not coincident at points502 and 504. Even if they are coincident at some time instant, since f₁does not equal f₂, they will not remain coincident for long data bursts.

[0030]FIG. 6 is a block diagram of a mobile terminal that implements theinvention. FIG. 6 illustrates a terminal with voice capability, such asa mobile telephone. In such a case, the first and second signals may becontain voice or data or a combination thereof. The two signals maycorrespond to one information stream (e.g. one voice call) or multipleinformation streams (e.g. two voice users in a three-way call). Thisillustration is an example only, and the invention works equally wellwith mobile terminals that are dedicated to communicating with text orother forms of data. As shown in FIG. 6, the terminal includes radioblock 601, a baseband logic block, 602, and an audio interface block,604. Within radio block 601, the receive and transmit information isconverted from and to the radio frequencies (RF) of the various carriertypes, and filtering is applied, as is understood in the art. Theterminal's antenna system, consisting of at least antenna 606, isconnected to the radio block. As previously mentioned, a terminal mayuse two antennas, and the optional second antenna, 601, is alsoconnected to the radio block. In baseband logic block 602, basic signalprocessing occurs, e.g., synchronization, channel coding, decoding andburst formatting. In this example, the baseband logic includes receiversubsystem 612, which performs interference canceling (I.C.), frequencyestimation (F.E.), and other functions, according to the invention. Thebaseband logic block can be implemented by one or more ASIC's, orperhaps by a digital signal processor (DSP). Audio interface block 604handles voice as well as analog-to-digital (A/D) and D/A processing.Processor and control logic block 608, coordinates the aforedescribedblocks and also plays an important role in controlling the humaninterface components (not shown) such as a keypad and liquid crystaldisplay (LCD).

[0031] Program code, often in the form of microcode is stored in memory603 and controls the operation of the terminal through the processor andcontrol logic. Memory 603 in this embodiment also stores any known setsof frequency offsets so that they can be accessed and used according tothe invention. The mobile terminal illustrated in FIG. 6 interfaces to asmart card identity module (SIM), 611, through a smart card readerinterface. The interconnection between the processor and control logic,memory, and SIM is depicted schematically. The interface is often aninternal bus. Also, any or all of these components may be discrete,implemented by multiple components, or integrated together in a singleor small number of semiconductor devices.

[0032] A mobile terminal implementation of the invention does not haveto be a traditional “cellular telephone type” of terminal, but mayinclude a cellular radiotelephone with or without a multi-line display;a personal communications system (PCS) terminal that may combine acellular radiotelephone with data processing, facsimile and datacommunications capabilities; a personal data assistant (PDA) that caninclude a radiotelephone, pager, Internet/intranet access, Web browser,organizer; and a conventional laptop and/or palmtop computer or otherappliance that includes a radiotelephone transceiver. Mobile terminalsare sometimes also referred to as “pervasive computing devices”.

[0033]FIG. 7 shows a base station system, 700, according to anembodiment of the invention. Base station system 700 includes at leastone base station, 702, which is setting up a communication with mobileterminal 703. In many embodiments, base stations 704, 705, and 706 areincluded in the base station system and are all similar or identical tobase station 702, and so some detail is omitted for these base stationsin FIG. 7. Base stations 704, 705, and 706 may take over communicationwith mobile terminal 703 as mobile terminal 703 moves through thesystem. Base station 702 includes two antennas, 707 and 709 to transmittwo user signals according to the invention. Transceiver apparatus, 710,includes the normal radio frequency components, processor, communicationlinks to a mobile switching center, etc., as is understood in the art.Finally, memory 712 stores the transmit frequency offset, 715, of basestation 702, and possibly frequency offsets of the neighboring basestations. The transceiver apparatus is operatively connected to theantennas and memory 712. The antennas transmit the signals atsubstantially the same bandwidth and carrier frequency. The memory mayalso contain at least some computer program code, 717, that operates thebase station.

[0034] In the embodiment of FIG. 7, it is assumed that a base station,in this example base station 702, establishes a set of frequency offsetsto use with mobile terminals. The value of the frequency offsets may beestablished directly, or impliedly by exchanging or transmitting valuesfor f₁ and f₂, as previously referred to. In a system with multipleneighboring base stations, the base stations may have frequency offsetsprovisioned locally, remotely, or they may negotiate the frequencyoffsets. In the illustrated embodiment, the set of frequency offsets orsets of frequency offsets are communicated to mobile terminals over acontrol channel, as shown at 716 as part of the process of establishingthe frequency offset or offsets to be used. When a plurality offrequency offsets are transmitted over a control channel to initiatecommunications, each unique set of transmit frequency offsetscorresponds to one of the plurality of base stations that make up thebase station system. Thus, a mobile station can identify each basestation by its unique set of offsets.

[0035] Computer program code elements of the invention may be embodiedin hardware and/or in software (including firmware, resident software,micro-code, etc.). These elements may take the form of a computerprogram product, which can be embodied by a computer-usable orcomputer-readable storage medium having computer-usable orcomputer-readable program instructions, “code” or a “computer program”embodied in the medium for use by or in connection with hardware such asthe base station transceiver apparatus. Such a medium is pictured inFIG. 7 as memory 712. In the context of this document, a computer-usableor computer-readable medium may be any medium that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer-usable or computer-readable medium may be, for example butnot limited to, an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system, apparatus, device, or propagationmedium such as the Internet. Note that the computer-usable orcomputer-readable medium could even be paper or another suitable mediumupon which the program is printed, as the program can be electronicallycaptured, via, for instance, optical scanning of the paper or othermedium, then compiled, interpreted, or otherwise processed in a suitablemanner.

[0036] Specific embodiments of an invention are described herein. One ofordinary skill in the telecommunication arts will quickly recognize thatthe invention has other applications in other environments. In fact,many embodiments and implementations are possible. The appended claimsare not intended to limit the scope of the invention to the specificembodiments described above. In addition, the recitation “means for” isintended to evoke a means-plus-function reading of an element in aclaim, whereas, any elements that do not specifically use thatrecitation are not intended to be read as means-plus-function elements,even if they otherwise include the word “means”.

What is claimed is:
 1. A method of demodulating first and secondsignals, the method comprising: receiving the first and second signalsfrom first and second associated transmit antennas at substantially thesame bandwidth and substantially the same carrier frequency; accessing aknown set of frequency offsets between the first and second signals andthe carrier frequency; and detecting symbols in the first and secondsignals using frequency estimates for the first and second symbols basedat least on part on the known set of frequency offsets.
 2. The method ofclaim 1 wherein the first and second signals are locked in carrierfrequency.
 3. The method of claim 1 further comprising applyingautomatic frequency control (AFC) to the first and second signals. 4.The method of claim 3 wherein the applying of the AFC further comprises:applying AFC to one of the first and second signals while canceling theother of the first and second signals; and applying AFC to the other ofthe first and second signals while canceling the of the first and secondsignals.
 5. The method of claim 1 further comprising applyingmaximum-likelihood frequency estimation to the first and second signals.6. The method of claim 1 further comprising applying a phase-locked loop(PLL) to the first and second signals.
 7. The method of claim 1 whereinthe known set of frequency offsets is one of a plurality of sets offrequency offsets, each associated with a plurality of correspondingbase stations so that a base station that corresponds to the first andsecond signals can be identified from among the plurality ofcorresponding base stations.
 8. The method of claim 2 wherein the knownset of frequency offsets is one of a plurality of sets of frequencyoffsets, each associated with a plurality of corresponding base stationsso that a base station that corresponds to the first and second signalscan be identified from among the plurality of corresponding basestations.
 9. The method of claim 3 wherein the known set of frequencyoffsets is one of a plurality of sets of frequency offsets, eachassociated with a plurality of corresponding base stations so that abase station that corresponds to the first and second signals can beidentified from among the plurality of corresponding base stations. 10.The method of claim 4 wherein the known set of frequency offsets is oneof a plurality of sets of frequency offsets, each associated with aplurality of corresponding base stations so that a base station thatcorresponds to the first and second signals can be identified from amongthe plurality of corresponding base stations.
 11. The method of claim 5wherein the known set of frequency offsets is one of a plurality of setsof frequency offsets, each associated with a plurality of correspondingbase stations so that a base station that corresponds to the first andsecond signals can be identified from among the plurality ofcorresponding base stations.
 12. The method of claim 6 wherein the knownset of frequency offsets is one of a plurality of sets of frequencyoffsets, each associated with a plurality of corresponding base stationsso that a base station that corresponds to the first and second signalscan be identified from among the plurality of corresponding basestations.
 13. Apparatus for demodulating first and second signals, theapparatus comprising: means for receiving the first and second signalsfrom first and second associated transmit antennas at substantially thesame bandwidth and substantially the same carrier frequency; means forstoring a known set of frequency offsets between the first and secondsignals and the carrier frequency; and means for detecting symbols inthe first and second signals using frequency estimates for the first andsecond symbols based at least on part on the known set of frequencyoffsets.
 14. The apparatus of claim 13 further comprising means forapplying automatic frequency control (AFC) to the first and secondsignals.
 15. The apparatus of claim 13 further comprising means forapplying maximum-likelihood frequency estimation to the first and secondsignals.
 16. The apparatus of claim 13 comprising means for applying aphase-locked loop (PLL) to the first and second signals.
 17. Theapparatus of claim 13 wherein the means for storing a known set offrequency offsets is operable to store a plurality of sets of frequencyoffsets, each associated with a plurality of corresponding basestations, and further comprising means for identifying a base stationfrom among the plurality of corresponding base stations based on theknown set of frequency offsets.
 18. The apparatus of claim 14 whereinthe means for storing a set of known frequency offsets is operable tostore a plurality of sets of frequency offsets, each associated with aplurality of corresponding base stations, and further comprising meansfor identifying a base station from among the plurality of correspondingbase stations based on the known set of frequency offsets.
 19. Theapparatus of claim 15 wherein the means for storing a set of knownfrequency offsets is operable to store a plurality of sets of frequencyoffsets, each associated with a plurality of corresponding basestations, and further comprising means for identifying a base stationfrom among the plurality of corresponding base stations based on theknown set of frequency offsets.
 20. The apparatus of claim 16 whereinthe means for storing a set of known frequency offsets is operable tostore a plurality of sets of frequency offsets, each associated with aplurality of corresponding base stations, and further comprising meansfor identifying a base station from among the plurality of correspondingbase stations based on the known set of frequency offsets.
 21. A mobileterminal comprising: at least one antenna; a radio block connected tothe at least one antenna, the radio block operable to transmit signals,and also to receive first and second signals from first and secondassociated associate transmit antennas at substantially the samebandwidth and substantially the same carrier frequency; processing andcontrol logic for controlling the operation of the mobile terminal; amemory connected to the processing and control logic, the memory forstoring a known set of frequency offsets between the first and secondsignals and the carrier frequency; and baseband logic operativelyconnected to the radio block, the processing and control logic and thememory, the baseband logic operable to detect symbols in the first andsecond signals using frequency estimates for the first and secondsignals based at least on part on the known set of frequency offsets.22. The mobile terminal of claim 21 wherein the at least one antennafurther comprises two antennas.
 23. The mobile terminal of claim 21wherein the baseband logic is further operable to apply automaticfrequency control (AFC) to the first and second signals.
 24. The mobileterminal of claim 21 wherein the baseband logic is further operable toapply maximum-likelihood frequency estimation to the first and secondsignals.
 25. The mobile terminal of claim 21 wherein the baseband logicis further operable to apply a phase-locked loop (PLL) to the first andsecond signals.
 26. The mobile terminal of claim 21 wherein the memoryand processing and control logic are operable to cause the memory tostore a plurality of sets of frequency offsets, each associated with aplurality of corresponding base stations, so that the mobile terminalcan distinguish a particular base station from among the plurality ofcorresponding base stations based on the known set of frequency offsets.27. The mobile terminal of claim 22 wherein the memory and processingand control logic are operable to cause the memory to store a pluralityof sets of frequency offsets, each associated with a plurality ofcorresponding base stations, so that the mobile terminal can distinguisha particular base station from among the plurality of corresponding basestations based on the known set of frequency offsets.
 28. The mobileterminal of claim 23 wherein the memory and processing and control logicare operable to cause the memory to store a plurality of sets offrequency offsets, each associated with a plurality of correspondingbase stations, so that the mobile terminal can distinguish a particularbase station from among the plurality of corresponding base stationsbased on the known set of frequency offsets.
 29. The mobile terminal ofclaim 24 wherein the memory and processing and control logic areoperable to cause the memory to store a plurality of sets of frequencyoffsets, each associated with a plurality of corresponding basestations, so that the mobile terminal can distinguish a particular basestation from among the plurality of corresponding base stations based onthe known set of frequency offsets.
 30. The mobile terminal of claim 25wherein the memory and processing and control logic are operable tocause the memory to store a plurality of sets of frequency offsets, eachassociated with a plurality of corresponding base stations, so that themobile terminal can distinguish a particular base station from among theplurality of corresponding base stations based on the known set offrequency offsets.
 31. A method of transmitting first and second signalsfor discrimination by a mobile terminal, the method comprising:establishing a known set of frequency offsets to be applied between thefirst and second signals and substantially the same carrier frequency,the known set of frequency offsets being known at the mobile terminal;applying the known set of frequency offsets to the first and secondsignals; upconverting the first and second signals for transmission atthe carrier frequency and at substantially the same bandwidth; andtransmitting the first and second signals to enable discrimination bythe mobile terminal based at least in part on the set of known frequencyoffsets.
 32. The method of claim 31 wherein the establishing of theknown set of frequency offsets further comprises transmitting the knownset of frequency offsets to the mobile terminal over a control channel.33. The method of claim 32 wherein the establishing of the known set offrequency offsets further comprises transmitting a plurality of sets offrequency offsets to the mobile terminal over the control channel,wherein the plurality of sets of frequency offsets correspond to aplurality of base stations.
 34. The method of claim 31 wherein the firstand second signals are locked together in carrier frequency.
 35. Themethod of claim 32 wherein the first and second signals are lockedtogether in carrier frequency.
 36. The method of claim 33 wherein thefirst and second signals are locked together in carrier frequency. 37.Apparatus for transmitting first and second signals for discriminationby a mobile terminal, the apparatus comprising: means for establishing aknown set of frequency offsets between the first and second signals andsubstantially the same carrier frequency, the set of known frequencyoffsets being known at the mobile terminal; means for storing the knownset of frequency offsets; means for applying the known set of frequencyoffsets to the first and second signals; and means for transmitting thefirst and second signals at the carrier frequency and at substantiallythe same bandwidth to enable discrimination by the mobile terminal basedat least in part on the known set of frequency offsets.
 38. Theapparatus of claim 37 further comprising means for sending the knownfrequency offset to the mobile terminal over a control channel.
 39. Theapparatus of claim 37 wherein the means for storing further comprisesmeans to store a plurality of frequency offsets corresponding to aplurality of base stations
 40. The apparatus of claim 38 wherein themeans for storing further comprises means to store a plurality offrequency offsets corresponding to a plurality of base stations, andwherein the means for sending further comprises means to send theplurality of frequency offsets to the mobile terminal over the controlchannel.
 41. A base station system for use in mobile communicationsystem, the base station system comprising at least one base station,which further comprises: a memory for storing at least one set oftransmit frequency offsets from substantially the same carrierfrequency; at least two antennas; and transceiver apparatus operativelyconnected to the memory and the at least two antennas, the transceiverapparatus operable to establish and apply a known set of frequencyoffsets from the at least one set of transmit frequency offsets to firstand second signals, the known set of frequency offsets being known to atleast one mobile terminal, and to transmit the first and second signalsvia the at least two antennas at the carrier frequency and atsubstantially the same bandwidth for discrimination by the at least onemobile terminal based at least in part on the known set of frequencyoffsets.
 42. The base station system of claim 41 wherein the at leastone base station comprises a plurality of base stations, and the atleast one transmit frequency offset comprises a plurality of transmitfrequency offsets, each corresponding to one of the plurality of basestations.
 43. The base station system of claim 41 wherein the knownfrequency offset is established at least in part by sending the knownfrequency offset to the mobile terminal on a control channel.
 44. Thebase station system of claim 42 further operable to send the pluralityof transmit frequency offsets to the mobile terminal over a controlchannel.
 45. The base station system of claim 41 wherein carrierfrequencies for the first and second signals are locked together. 46.The base station system of claim 42 wherein carrier frequencies for thefirst and second signals are locked together.
 47. The base stationsystem of claim 43 wherein carrier frequencies for the first and secondsignals are locked together.
 48. The base station system of claim 44wherein carrier frequencies for the first and second signals are lockedtogether.
 49. A computer program product comprising a computer programfor transmitting first and second signals for discrimination by a mobileterminal, the computer program comprising: instructions for establishinga known set of frequency offsets between the first and second signalsand substantially the same carrier frequency, the known set of frequencyoffsets being known at the mobile terminal; instructions for storing theknown set of frequency offsets; instructions for applying the known setof frequency offsets to the first and second signals; and instructionsfor transmitting the first and second signals at the carrier frequencyand at substantially the same bandwidth to enable discrimination by themobile terminal based at least in part on the known set of frequencyoffsets.
 50. The computer program product of claim 49 wherein thecomputer program further comprises instructions for sending the knownset of frequency offsets to the mobile terminal over a control channel.51. The computer program product of claim 49 wherein the instructionsfor storing further comprise instructions for storing a plurality ofknown sets of frequency offsets corresponding to a plurality of basestations
 52. The computer program product of claim 50 wherein theinstructions for storing further comprise instructions for storing aplurality known sets of frequency offsets corresponding to a pluralityof base stations, and wherein the instructions for sending furthercomprise instruction for sending the plurality of known sets offrequency offsets to the mobile terminal over the control channel.
 53. Amethod of communicating first and second signals between a base stationand a mobile terminal, the method comprising: establishing a known setof frequency offsets to be applied between the first and second signalsand substantially the same carrier frequency; sending the known set offrequency offsets to the mobile terminal; applying the known set offrequency offsets to the first and second signals; upconverting thefirst and second signals to the carrier frequency; transmitting thefirst and second signals at the carrier frequency and at substantiallythe same bandwidth to enable discrimination by the mobile terminal; anddetecting symbols in the first and second signals at the mobile terminalusing frequency estimates for the first and second symbols based atleast on part on the known set of frequency offsets.
 54. The method ofclaim 53 wherein the first and second signals are locked in carrierfrequency at the base station.
 55. The method of claim 53 furthercomprising applying automatic frequency control (AFC) to the first andsecond signals at the mobile terminal.
 56. The method of claim 53further comprising applying maximum-likelihood frequency estimation tothe first and second signals at the mobile terminal.
 57. The method ofclaim 53 further comprising applying a phase-locked loop (PLL) to thefirst and second signals at the mobile terminal.
 58. The method of claim54 wherein the known set of frequency offsets is one of a plurality ofsets of frequency offsets, each associated with one of a plurality ofcorresponding base stations so that the base station can be identifiedfrom among the plurality of corresponding base stations.
 59. The methodof claim 55 wherein the known set of frequency offsets is one of aplurality of sets of frequency offsets, each associated with one of aplurality of corresponding base stations so that the base station can beidentified from among the plurality of corresponding base stations. 60.The method of claim 56 wherein the known set of frequency offsets is oneof a plurality of sets of frequency offsets, each associated with one ofa plurality of corresponding base stations so that the base station canbe identified from among the plurality of corresponding base stations.61. The method of claim 57 wherein the known set of frequency offsets isone of a plurality of sets of frequency offsets, each associated withone of a plurality of corresponding base stations so that the basestation can be identified from among the plurality of corresponding basestations.
 62. A system for communicating first and second signalsbetween a base station and a mobile terminal, the system comprising:means for establishing a known set of frequency offsets to be appliedbetween the first and second signals and substantially the same carrierfrequency; means for sending the known set of frequency offsets to themobile terminal; means for applying the known set of frequency offsetsto the first and second signals; means for upconverting the first andsecond signals to the carrier frequency; means for transmitting thefirst and second signals at the carrier frequency and at substantiallythe same bandwidth to enable discrimination by the mobile terminal; andmeans for detecting symbols in the first and second signals at themobile terminal using frequency estimates for the first and secondsymbols based at least on part on the known set of frequency offsets.63. The system of claim 62 further comprising means for storing aplurality of sets of frequency offsets corresponding to a plurality ofbase stations so that the mobile terminal can identify the base stationfrom among the plurality of base stations.
 64. The system of claim 62wherein the means for detecting further comprises means for applyingautomatic frequency control (AFC) to the first and second signals at themobile terminal.
 65. The system of claim 62 wherein the means fordetecting further comprises means for applying maximum-likelihoodfrequency estimation to the first and second signals at the mobileterminal.
 66. The system of claim 62 wherein the means for detectingfurther comprises means for applying a phase-locked loop (PLL) to thefirst and second signals at the mobile terminal.
 67. The system of claim63 wherein the means for detecting further comprises means for applyingautomatic frequency control (AFC) to the first and second signals at themobile terminal.
 68. The system of claim 63 wherein the means fordetecting further comprises means for applying maximum-likelihoodfrequency estimation to the first and second signals at the mobileterminal.
 69. The system of claim 63 wherein the means for detectingfurther comprises means for applying a phase-locked loop (PLL) to thefirst and second signals at the mobile terminal.